P2.B.3Hebb's theory of learning and neural plasticity; the formation of cell assemblies

Donald Hebb (1949) proposed one of the most influential ideas in neuroscience: that learning happens because of physical changes in the connections between neurons. The slogan version is "neurons that fire together, wire together" — Hebb's law.

Hebb's central claims

1. Cell assemblies. When a stimulus is repeatedly experienced, the same group of neurons fires together. Over time, the synaptic connections between them strengthen, forming a cell assembly — a circuit that represents that stimulus or idea.
2. Long-term potentiation (LTP). Repeatedly activating two connected neurons together makes the postsynaptic neuron more responsive to future signals from the presynaptic neuron. This is the cellular basis of memory.
3. Phase sequences. Multiple cell assemblies can chain together into a phase sequence, representing a complex thought or memory (e.g. the route to school activates "front door → road → bus stop → school gates").

Neural plasticity

Plasticity is the brain's ability to physically change in response to experience. Modern research has confirmed Hebb's prediction in three forms:

• Synaptic plasticity — strengthening (LTP) or weakening (LTD) of existing synapses.
• Structural plasticity — growth of new dendritic branches and new synapses with experience; pruning of unused connections.
• Functional plasticity — entire brain regions can take on new roles after damage, especially in young brains (e.g. children with hemispherectomies recovering language with the remaining hemisphere).

Evidence

• Maguire et al. (2000) — London taxi drivers who had passed "the Knowledge" had a larger posterior hippocampus (the brain region associated with spatial navigation) than non-taxi-driver controls. The longer they had been driving, the bigger the difference. Hippocampal grey matter had grown in response to the years of navigation practice.
• Animal studies (e.g. Rosenzweig & Bennett, 1972) showed rats raised in enriched environments — toys, ramps, other rats — developed thicker cortex and more synapses than rats in standard cages. Direct evidence that experience reshapes the brain.
• Stroke recovery — patients can regain lost function (e.g. speech, movement) through intensive therapy, as nearby healthy regions take over the role of the damaged area.

Implications for learning

Hebb's theory provides the biological basis for practice and rehearsal:

• Spaced repetition (revisiting material across days) drives LTP and stable cell assemblies.
• Active retrieval is more effective than passive re-reading because retrieval activates and strengthens the memory's cell assembly.
• "Use it or lose it" — unused synapses are pruned over time.

Limitations

• Hebb's original theory was a theoretical proposal (1949); the cellular evidence (LTP discovered in 1973 by Bliss and Lømo) came decades later.
• Plasticity is greater in younger brains; not all aspects of Hebb's theory generalise across the lifespan.
• A simplified picture: learning involves whole networks, neuromodulators, glial cells and protein synthesis, not just paired neuron firing.